A metal material such as a zinc-plated steel sheet material, an aluminum material or the like is oxidized and corroded by oxygen and moisture in air, and by ions contained in moisture, etc. As a method for preventing such corrosion, there is a method for forming a chromate coating film through contact of a metal surface with a chromium-containing treating liquid such as chromium chromate, chromium phosphate or the like. The coating film formed according to the chromate treatment has excellent corrosion resistance and coating film adhesiveness, but the treatment liquid contains harmful hexavalent chromium and is problematic in that wastewater treatment takes a lot of trouble and cost. In addition, the coating film formed according to the treatment also contains hexavalent chromium, and therefore environmental and safety problems are pointed out.
Accordingly, aqueous liquid compositions for metal surface treatment and chemical conversion treatment agents not containing a chromate (chromium-free) but having corrosion resistance on the same level as that of already-existing chromate chemical conversion coating films have been proposed (for example, see PTLs 1, 2).
The metal surface treatment agent in PTL 1 is a chromium-free metal surface treatment agent containing a vanadium compound (A), a metal compound (B) containing a metal selected from cobalt, nickel, zinc, magnesium, aluminium, calcium, strontium, barium and lithium, and optionally a metal compound (C) containing zirconium, titanium, molybdenum, tungsten, manganese and cerium, which can impart excellent corrosion resistance, alkali resistance and interlayer adhesiveness to a metal material.
The metal surface treatment agent in PTL 2 is a metal surface treatment agent containing one or more Group-4 transition metal compounds (a) selected from a Zr compound capable of releasing zirconyl ion (ZrO2+) in an aqueous solution and a Ti compound capable of releasing a titanyl ion (TiO2+) in an aqueous solution, and an organic compound (b) having two or more of at least one functional group selected from a hydroxyl group, a carboxyl group, a phosphonic acid group, a phosphoric acid group and a sulfonic acid group, in one and the same molecule, and is a chromium-free metal surface treatment agent capable of imparting high adhesiveness in such a level that, even when a resin coating film formed after chemical conversion coating film formation is processed in a severe forming process of deep-drawing or the like, the resin coating film is not peeled off.
Both the metal surface treatment agents in PTLs 1 and 2 may contain an aqueous resin that may be soluble in water or dispersible in water.
On the other hand, since the proposal in PTL 3, it is known that a molten zinc-aluminum-magnesium plated steel sheet using a plating bath containing suitable amounts of aluminum and magnesium in zinc is excellent in corrosion resistance.